ObjectiveTo test if serially sampled determinations of the contact system proteins and factor V have prognostic value for death in patients who develop the systemic inflammatory response syndrome.DesignProspective, observational study with sequential measurements in an inception cohort.SettingMedical intensive care unit (ICU) in a community hospital.PatientsOver a 1-yr period, a population base sample of 23 patients was selected from all ICU admissions who met established criteria for the systemic inflammatory response syndrome.InterventionsNone.Measurements and Main ResultsComponents of the contact system, factor XII, prekallikrein, high-molecular-weight kininogen, factor XI, alpha2-macroglobulin-kallikrein complexes and factor V values were measured in plasma samples collected serially (day of admission, and at 2, 12, 24, 48 and/or 72 hrs or at discharge). Data were analyzed to determine if admission values or serially obtained values within 48 hrs were useful in predicting outcome. Fourteen patients survived and nine died.At admission, in all patients, assay values indicated that prekallikrein, high-molecular-weight kininogen, and factor V were significantly lower than normal (as observed in a range of 20 to 23 healthy adults), alpha2-macroglobulin-kallikrein complexes were higher than normal, while concentrations of factor XII and factor XI were in the normal range. No differences were detected in the admission values between survivors and nonsurvivors, nor between patients with positive or negative blood cultures. However, subsequent values demonstrated a difference in values between survivors and nonsurvivors. Survivors showed improvement in high molecular weight kininogen values and higher than normal factor V values, as compared with nonsurvivors.ConclusionsLow or persistently low serial factor XII, high-molecular-weight kininogen and factor V values are associated with a poor prognosis, whereas high or increasing values of factor XII, high-molecular-weight kininogen, prekallikrein, and factor V all correlate with a favorable outcome.(Crit Care Med 1995; 23:41-51)